Surfactants are well known materials that can be generally described as having a hydrophobic moiety and a hydrophilic group per molecule. A wide variety of these materials are known and they have numerous uses such as emulsifiers, detergents, dispersants and solubilizing agents in the fields of cosmetics, textile treatment, industrial and personal cleaning preparations, corrosion inhibitors and the like.
Surfactants can be anionic, cationic, amphoteric or nonionic. Anionic surfactants carry a negative charge on the hydrophilic portion, usually in the form of a carboxylate, phosphate, sulfate or sulfonate. These surfactants find use in emulsion polymerization as well as in agricultural chemicals, personal care and household products, industrial and institutional cleaners. They function as emulsifiers, cleaners, wetting agents, foaming and frothing agents for shampoos, car washes, carpet shampoos, hand dishwashing, latex foaming, oil recovery and other industrial uses. See, generally Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, John Wiley and Sons, New York, 1997, Vol. 23, pp. 478-541.
Cationic surfactants carry a positive charge on the hydrophilic moiety, most generally in the form of an amine or quaternary ammonium salt. These surfactants are used, for example, as textile softeners, conditioning agents, dispersants, emulsifiers, wetting agents, sanitizers, dye-fixing agents, foam stabilizers and corrosion inhibitors. See, for example, Drew et al., U.S. Pat. No. 3,001,945, issued Sep. 26, 1961.
Nonionic surfactants carry no charge when they are dissolved in an aqueous medium. The hydrophilicity in these surfactants is the result of hydrogen bonding interactions between the surfactant and the aqueous medium. Nonionic surfactants are generally derivatives of polyoxyethylenes.
Surfactants are frequently employed in the treatment of wastewater and waste products from various sources to stabilize emulsions. Surfactant stabilized emulsions can lead to sludge generation, high costs for disposal, recovery of recyclable materials, and other problems. Emulsions of waste products can create significant environmental problems because they are usually extremely difficult to convert into more valuable, useful or innocuous products. Further, they are usually not readily susceptible to emulsion breaking techniques and separation into aqueous and organic phases. As the incineration of the organic waste component of an emulsion requires the removal of substantial amounts of water, the inability to separate the emulsion into aqueous and burnable fractions further complicates production and recycling efforts.
Efforts have been directed to producing compounds having surfactant properties that can be xe2x80x9cturned offxe2x80x9d as a result of a structural change in the surfactant, producing a class of surfactants that are xe2x80x9cchemodegradablexe2x80x9d (See, Jaeger, Supramolecular Chemistry 5: 27-30 (1995); Holmberg, Curr. Opin. Colloid Interface Sci. 1: 572-579 (1996)). xe2x80x9cChernodegradable surfactantsxe2x80x9d are surface-active compounds that can undergo specific chemical reactions and thereby lose their surface activity (Sokolowksi et al., J. Am. Oil Chem. Soc. 69: 633-638 (1992)).
Chemodegradable surfactants have attracted substantial interest in the recent past for several reasons (Holmberg, supra). For example, the capability to transform water-soluble molecules from surface-active states to surface-inactive ones, can be used to break emulsions or foams following their use in lubricant formulations (White, Chemical Processing 60: 32-32 (1997)). Chemodegradable surfactants are also useful as emulsifiers in emulsion polymerizations because their subsequent conversion to surface inactive-states can be exploited to eliminate the presence of charged surfactant head groups at surfaces of latex particles, increasing the water resistance of resultant polymer films. (Mezger, et al., Progress in Organic Coatings 29, 147-157 (1996)) Chemodegradable surfactants are also potentially useful in procedures such as those used for the isolation, purification and reconstitution of proteins in lipid environments. The surfactants can be used to solubilize membrane proteins, but can then be transformed into fragments that are readily displaced by natural phospholipids (Cuomo et al., J. Org. Chem. 45:4216-4219 (1980).
Although the known chemodegradable surfactants exhibit interesting properties, they are not ideal for a number of reasons. In general, these agents are hampered by their slow degradation upon contact with the cleavage reagent, and the need to use great excesses of the cleavage reagent to achieve quantitative cleavage. Other agents are rendered of comparatively little use due to their low water solubility, instability in solutions that are acidic or basic, and the need to be stored and used under restrictive conditions.
Chemodegradable surfactants comprising a disulfide moiety have been investigated. The disulfide-containing degradable surfactants are generally poorly water-soluble. Although two water-soluble, disulfide-containing, cationic surfactants have been reported, these compounds are rapidly degraded in aqueous solutions having a pH greater than 8. Furthermore, these agents are not described as being degradable by either oxidation or reduction, rather these agents are described as interacting with, and binding to, kerastin fibers through a disulfide exchange reaction. Moreover, these agents are not used to reversibly form or to disrupt emulsions (Pinazo, A. et al., J. Am. Oil Chem. Soc. 70:37-42 (1993)).
An ideal chemodegradable surfactant should meet a number of criteria. First, the incorporation of a chemodegradable group in the surfactant should not unduly complicate the synthesis of the surfactant. Also, the surface properties of chemodegradable surfactants should not be compromised relative to classical surfactants prior to their transformation into less surface-active species. Furthermore, the chemodegradable surfactants must be water-soluble and stable under a range of conditions (e.g., broad pH range, light, heat, etc.). Moreover, the transformation of the chemodegradable surfactant into its less surface-active state should be effected without requiring a large excess of a transforming agent.
Presently available chemodegradable surfactants do not meet each of these criteria, however, quite surprisingly, the present invention provides surfactants meeting these criteria and methods of using these surfactants.
It has now been discovered that surfactants containing a disulfide bond within their framework, which can be cleaved or made more hydrophilic by an oxidant, meet the criteria set forth above. The present surfactants are substantially water-soluble and thus, are useful for reversibly stabilizing water-based emulsions. The reversible emulsions can be disrupted following the cleavage or increase in hydrophilicity of the disulfide bond via an oxidative process.
The compounds of the invention possess a number of advantages over known chemodegradable surfactants. For example, the present surfactants are quickly cleaved by inexpensive, readily-available and mild oxidants, such as sodium hypochlorite. Often the cleavage is complete in five minutes or less. Moreover, the cleavage process is a one-step process and, unlike prior art acid- and base-cleavable surfactants, the process does not require any subsequent neutralization steps. Furthermore, the molecules of the invention are easily assembled and characterized using art-recognized organic synthetic methods.
Thus, in a first aspect, the present invention provides a substantially water-soluble surfactant according to Formula (I): 
wherein: P1 and P2 are members independently selected from the group consisting of anionic groups, cationic groups, neutral hydrophilic groups and zwitterionic groups; and R1 and R2 are members independently selected from C1 to C24 straight-chain alkyl, C1 to C24 branched-chain alkyl, C1 to C24 straight-chain substituted alkyl and C1 to C24 branched-chain substituted alkyl.
In a second aspect, the present invention provides a substantially water-soluble surfactant according to Formula (II): 
wherein: P1 and P2 are members independently selected from the group consisting of anionic groups, cationic groups, neutral hydrophilic groups and zwitterionic groups; m is a number from 0 to 24, inclusive and n is a number from 0 to 24, inclusive.
In a third aspect, the present invention provides a substantially water-soluble surfactant according to Formula (III):
P1R1xe2x80x94Sxe2x80x94Sxe2x80x94R2P2xe2x80x83xe2x80x83(III)
wherein: P1 and P2 are members independently selected from the group consisting of anionic groups, cationic groups, neutral hydrophilic groups and zwitterionic groups; R1 and R2 are members independently selected from C1 to C24 straight-chain alkyl, C1 to C24 branched-chain alkyl, C1 to C24 straight-chain substituted alkyl and C1 to C24 branched-chain substituted alkyl; and the surfactant remains at least 80% intact after 24 hours in aqueous solution at pH 8 or greater.
In a fourth aspect, the invention provides an aqueous solution comprising a surfactant according to Formula (I): 
wherein:
P1 and P2 are members independently selected from the group consisting of anionic groups, cationic groups, neutral hydrophilic groups and zwitterionic groups; R1 and R2 are members independently selected from C1 to C24 straight-chain alkyl, C1 to C24 branched-chain alkyl, C1 to C24 straight-chain substituted alkyl and C1 to C24 branched-chain substituted alkyl; and the solution is switchable between a first state and a second state by oxidation of the disulfide group of the surfactant.
In a fifth aspect, the invention provides a method for increasing surface tension of an aqueous solution of a surfactant comprising a disulfide moiety, the method comprising: contacting the solution with an oxidant, thereby oxidizing the disulfide moiety.
In a sixth aspect, the invention provides a method for disrupting an emulsion comprising: (a) water; (b) a substantially water-insoluble material; and (c) a substantially water-soluble surfactant comprising a disulfide moiety, the method comprising: contacting the emulsion with an oxidant, thereby oxidizing the disulfide moiety.
In a seventh aspect, the invention provides a method for switching a mixture between an emulsified mixture and a non-emulsified mixture. The emulsified mixture comprises: (a) water; (b) a substantially water-insoluble material; and (c) a substantially water-soluble surfactant comprising a disulfide moiety. The non-emulsified mixture comprises: (a) water; (b) a substantially water-insoluble material; and (c) a substantially water-soluble surfactant comprising a thiol moiety. The method comprises; (a) contacting the emulsified mixture with a reducing agent, thereby cleaving the disulfide and forming the non-emulsified mixture; and (b) contacting the non-emulsified mixture with an oxidant, thereby producing a disulfide and forming the emulsified mixture.
In an eighth seventh aspect, the invention provides a method for producing a transient reduction in the surface tension of a solution or emulsion comprising a disulfide-containing surfactant of the invention. The method comprises contacting the surfactant with a reducing agent.
Other features, objects and advantages of the invention and its preferred embodiments will be apparent from the detailed description which follows.